Crucible

ABSTRACT

The crucible is formed of compacted graphite or vitreous carbon and has an interior tapered toward its nucleation point. Single and multiport types are described for growing one or more crystals similtaneously. The crucible is useful for growing laser quality crystals of alkaline earth and rare earth fluorides, including mixed rare earth fluorides and mixed rare earthalkaline earth fluorides which are free from oxides and oxyfluorides.

United States Patent 1 Robinson et al.

I4 1 Mar. 12, 1974 CRUCIBLE Division of Ser. No. 627,355, March 31,1967, Pat. No. 3,649,552.

Assignee:

U.S. Cl. 23/292, 23/273 R, 23/277 R,

- 164/60, 432/263, 266/39, 23/252 A, 423/489 Int. Cl BOII 3/04 Field ofSearch 23/277 R, 273 R, 301 SP,

23/273 SP, 292; 148/15, 16;- 164/60; 432/263; 266/34 V, 39

References Cited UNITED STATES PATENTS Rocco 164/60 3,694,166 9/1972Kyle 23/273 R OTHER PUBLICATIONS Fisher, Modern Laboratory AppliancesCatalog No. 63 (1962), pp. 170 & 189.

Primary Examiner-James H. Ta yman, Jr. Attorney, Agent, or Firm-James K.Haskell; Lewis B. Sternfels; W, H. MacAllister [57] ABSTRACT Thecrucible is formed of compacted graphite or vitreous carbon and has aninterior tapered toward its nucleation point. Single and multiport typesare described for growing one or more crystals similt aneously. Thecrucible is useful for growing laser quality crystals of alkaline earthand rare earth fluorides, in-

cluding mixed rare earth fluorides and mixed rare earth-alkaline earthfluorides which are free from oxides and oxyfluorides.

7 Claims, 8 Drawing Figures I I14 I24 I M ./IIO I28/{ I 112 Izz fPATENTEI] MAR 12 I974 SHEEI 2 OF 2 8. ISL-$7 f Fig 7.

Fig. 8.

l CRUCIBLE CROSS-REFERENCE TO RELATED APPLICATION I BACKGROUND OF THEINVENTION The present invention relates to a crucible for growingcrystals. It is specifically useful in the preparation of optically purecrystals of alkaline earth and rare earth fluorides, including mixedrare earth fluorides and mixed rare earth-alkaline,earth fluorides oflaser quality free from oxides and oxyfluorides.

Laser devices require the use of crystals which are of ultra-highoptical purity since even minute impurities cause internal scatter andopaqueness in the crystal, thereby preventing the desired lightamplification. In

K recent years, interest has centered on crystals doped PRIOR ARTMethods of growing crystals have been limited in that only one crystalat a time could be grown. Because the growth of high quality crystals ismarkedly dependent upon temperature and uniformity of temperature withinthe crystal, the time for growing a single crystal encompassed a periodof time which can easily extend from a few days to as much as a fewweeks. Consequently,-it has not been previously possible to obtain alarge quantity production of crystals in the absence of a large numberof furnaces in which-the crystals may be grown. It is obvious,therefore, that the growth of a large number of crystals required acorrespondingly large investment in process equipment.

Furthermore, the crucibles used in prior processes to form such crystalsas laser crystals were formed for platinum or platinum lined withcarbon. When a platinum crucible is used, the formed crystal adheres tothe platinum which requires that the crucible be torn away from thecrystal so that the crystal may be obtained. Thus, such a cruciblebecomes unusable for further growth. When carbon is used as a liner fora platinum crucible and is exposed to such corrosive gasses as hydrogenfluoride, a reaction occurs at elevated temperatures to cause adiffusion of the carbon through the platinum. The platinum becomesbrittle although the fluoride crystal is not contaminated; however, thelifetime of the platinum with its'carbon liner is shortened.Furthermore, many prior processes utilized a closed crucible in whichthe crystal material and ambient atmosphere are contained so that, ifthe ambient atmosphere included hydrogen fluoride, noxious fumes wouldnot escape-therefrom. However, when the crystal was formed, the cruciblehad to be destroyed in order to remove the crystal.

' SUMMARY OF THE INVENTION The present invention overcomes these andother problems also by providing a very simple process means wherein amultiplicity of quality crystals may be formed at the same time.

In the growth of a crystal, a starting material may be loaded into acrucible which is formed with an open -end. The crucible with thestaring material therein is attached to a lowering rod in a crystalgrowth furnace and the temperature of thefurnace is slowly raised in adesired atmosphere above the melting point of the material to producemelt.

The crucible is so constructed as to aid contact of the atmosphere withthe melt, if needed. The crucible is thenlowered at a rate commensuratewith the growth rate of the crystal. At the end of the growth travelregion, crystal annealing usually takes place. The time required forsatisfactory annealing depends on the crystal material and the cruciblematerial as well as the length and diameter of the crystal. The crystalis slowly cooled to a specified temperature, and the furnace is cooledto room temperature over a suitable period of time.

Preferably, when used to grow the fluoride crystals, the crucibles aremade from graphite since neither a fluoride melt nor hydrogen fluorideatmosphere affects it. Other advantages of graphite are that thefluorides do not wet the graphite thus facilitating removal of thecrystal from the crucible and that the graphite is of sufficientmechanical stability so that no fine graphite materials 'are transferredas impurities to the crystal. The crucibles are formed with one or moreports so that one or more crystals may be grown singly orsimultaneously. Each portis closed at its bottom end and open at itsupper end. The diameterof each port is greatest at its open end anddecreases toward its closed end whereit terminates at a point tofacilitate nucleation of a single crystal. The decreasing diameter, inconjunction with the non-wetting characteristic of graphite, permitseasy removal of the crystal grown. The decreasing diameter also acts asa funnel to increase contact of hydrogen fluoride with the melt. It isto be understood, however, that the graphite crucibles will facilitateremoval of crystals regardless of the particular method of halidecrystal growth employed.

Any suitable furnace may be used for crystal growth.

Other aims and objects, as well asa more complete understanding of thepresent invention willappear from the following explanation of exemplaryembodiments and the accompanying drawings thereof, in.

which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 6 is a bottom view, taken alonglines 6 -6 of FIG. 2, of the multiport crucible;

FIG. 7 is a side elevational view of a second multiport crucible forgrowing several crystals simultaneously;

' and FIG. 8 is a view, taken along lines 8--8 of FIG. 7, of the secondembodiment of the multiport crucible.

DESCRIPTION OF THE PREFERRED EMBODIMENTS One use of the presentinvention is more fully described in the above-noted US. Pat. No.3,649,552.

The crucibles are formed entirely for vitreous carbon or a compactedgraphite since neither the corrosive nature of a fluoride melt nor ahydrogen fluoride atmosphere affects the graphite. In addition, thefluorides do not wet the graphite, thus making the removal of theproduct quite easy. Furthermore, carbon has a melting point which is atleast twice that of any of the crystals made by the inventive method andis insoluble therein. The graphite crucibles additionally have goodthermal characteristics such that the heat of the furnace may be applieduniformly throughout the crystals during their growth. The graphitecrucibles are relatively inexpensive, are easily machinable, and tend toresist shock. It is in partfor these reasons that it is possible toformmultipo'rt crucibles so that several crystals may be grownsimultaneously.

A single port crucible 110 is depicted in FIG. 1 and comprises a tubularportion 112, an entry portion 114, and an intermediate portion 116.Tubular portion 112 terminates in a point 118 for closure thereof. Thecrucible is provided with a bore 120 which varies according to theparticular portion to form inner walls 122, 124 and 126, respectively,of tubular portion 112, entry portion 114 and intermediate portion 116.Inner wall 112 of portion 112 is provided with a diameter at its upperend 128 which is greater than the diameter at its lower end 130. Thus,inner wall 122 tapers from its upper end to its lower end. Inner wall132 of point 118 converges to a point from lower end 130 of portion 122to serve as a nucleation point by which growth of a single crystal maybe accomplished. Inner wall 124 of portion 114 is cylindrical and isprovided with a diameter which is relatively larger than the diameter ofwalls 126 and 122. Inner wall 126 of portion 116 converges from innerwall 124 to the upper end 128 of inner wall 122 thus providing a gentletaper of bore 120.

Referring now to FIGS. 1-6, a multiport crucible 132 comprises a solidupper section 134, having an entry portion 136 and an intermediateportion 138, and a lower section 140 having tubular portions 142 andfins 144. Entry portion 136 is provided with an outer diameter which isgreater than that of lower section 140 and intermediate portion 138 hasan outer diameter which tapers from that of portion 136 to that ofsection 140. A tapped and internally threaded connecting means 146 iscentrally disposed within entry portion 136 and crucible 132 and formsan opening in upper face 148 of portion 136. An externally threaded rodis adapted to threadedly engage crucible 132 within connecting means 146to enable the positioning and movement of the multiport crucible withina crystal growing furnace.

As depicted in FIGS. 2-6, crucible 132 is adapted to permit simultaneousgrowth of six crystals in six individual components 150 of crucible 132.The internal construction of each component 150 is similar to that asillustrated with respect to FIG. 1 in such a manner that each tubularportion 142 is provided with an inner wall 152 which tapers from itsupper end toward its lower end, entry portion 136 is provided with aninner cylindrical wall 154 and intermediate portion 138 is provided witha tapered inner wall 156 which joins inner wall 154 with the upper endof inner wall 152. The lower end of inner wall 152 converges to anucleation point 158 within a point 160 of tubular portion 142.

Fins 144 extend downwardly from intermediate portion 138 and are formedwith curved suraces 162, as best seen in FIGS. 5 and 6, which are spacedfrom but concentric with tubular portion 142. Fins 144 are used to addsupport to crucible 132, to protect points 160 of tubular portions 142,and to prevent dissipation of heat from the tubular portions so that thegrowth and the stability of the crystals will be enhanced. A circularoutput 164, concentric with the axis of crucible 132, is placed interiorto tubular portions 142 and fins 144 to permit even distribution of heatto the crystals during the time of their formation.

A third embodiment (see FIGS. 7 and 8) comprises a multiport crucibleincluding four tubular components 172 having pointed extremities 174 atone end thereof and entry portions 176 at the other extremity thereof.Components 172 are joined at their upper ends by a web 178 (see FIG. 8)and a threaded nipple 180 extends from web 178 for engagement with arod. The inner walls 182 of each of the tubular components 172 aresimilar to inner wall 122 of tubular portion 112, as depicted in FIG. 1.

In all cases, those inner walls which are in contact with the crystalare tapered in order to facilitate removal of the crystal after havingbeen grown without necessitating the destruction of the crucible.

Although the invention has been described with reference to particularembodiments thereof, it should be realized that various changes andmodifications may be made therein without departing from the spirit andscope of the invention.

What is claimed is:

1. A crucible for growing crystals from starting materials comprising anelongate tube for receiving the starting materials and having:

internal bore means bounded at both ends by means defining openings,said opening means at a first of said ends being larger in dimensionthan the opening means at a second of said ends to provide said boremeans with a taper decreasing from said first end to said second end;and

a closed tapered pointed end at and terminating said second end of saidbore means.

2. A crucible as in claim 1 further including an entry portion providedwith opening means of greater dimension than that of said bore means atsaid first end and a connecting portion secured between said .entryportion and said tube at said first end, said connecting portion havinga tapered bore diminishing in dimension from the entry portion bore tothe first end.

3. A crucible as in claim 1 wherein said tube is constructed ofcompacted graphite.

4. A multiport crucible for simultaneously processing several cyrstalsincluding a plurality of elongate tubes for each of the severalcrystals, each said tube having internal bore means bounded by first andsecond open portions, and a closed pointed end terminating said secondopen portion, said bore means of each said tube v v 6 I being of greaterdimension at said first open portion between said entry ports and saidfirst open portions. than at said second open portion to form a taper insaid bore means, and means securing said tubes together at 6'. Acrucible as in claim 5 further including a pluralsaid first openportions. ity of fins secured to said entry section and partially en- 5.A crucible as in claim 4 further including an entry 5 closing saidtubes.

section having a plurality of entry ports, each of said 7. A crucible asin claim 6 formed from compacted ports having a dimension greater thanthat of each of graphite.

said first open portions, and passage means connected

2. A crucible as in claim 1 further including an entry portion providedwith opening means of greater dimension than that of said bore means atsaid first end and a connecting portion secured between said entryportion and said tube at said first end, said connecting portion havinga tapered bore diminishing in dimension from the entry portion bore tothe first end.
 3. A crucible as in claim 1 wherein said tube isconstructed of compacted graphite.
 4. A multiport crucible forsimultaneously processing several cyrstals including a plurality ofelongate tubes for each of the several crystals, each said tube havinginternal bore means bounded by first and second open portions, and aclosed pointed end terminating said second open portion, said bore meansof each said tube being of greater dimension at said first open portionthan at said second open portion to form a taper in said bore means, andmeans securing said tubes together at said first open portions.
 5. Acrucible as in claim 4 further including an entry section having aplurality of entry ports, each of said ports having a dimension greaterthan that of each of said first open portions, and passage meansconnected between said entry ports and said first open portions.
 6. Acrucible as in claim 5 further including a plurality of fins secured tosaid entry section and partially enclosing said tubes.
 7. A crucible asin claim 6 formed from compacted graphite.